Shielding for a vacuum type circuit interrupter



Get. 3, i967 G, PoLlNKo, JR., ETAL 3,345,484

SHIELDING FOR A VACUUM TYPE CIRCUIT INTERRUPTER Filed may 1o, 1965 FLUX/A/ CENTE/P 0F CO/VT/ICTREG//V GEORGE POUM/(UMR. JOSEPH I/l/. PORTER,

ATTORNEY United States Patent O 3,345,434 SHHELDING FOR A VACUUM TYPECIRCUIT INTERRUPTEUR George Polinko, Jr., West Chester, and `l'osepli W.Porter, Media, Pa., assignors t General Electric Company, a corporationof New York Filed May 10, 1965, Ser. No. 454,282 6 Claims. (Cl. 20D-144)This invention relates to a vapor-condensing metal shield for avacuum-type circuit interrupter and relates, more particularly, to meansfor reducing the eddy currents induced in such a shield by a varyingmagnetic field that extends axially of the interrupter.

The usual vacuum type interrupter comprises a pair of relatively movablecontacts, or electrodes, that can be separated to establish an arcinggap therebetween across which an arc is formed. The arc vaporizes someof the electrode material to create a local atmosphere through whichcurrent flows until about the time a natural current zero is reached.When the current zero point is reached, the arc vanishes, and the usualrecovery voltage transient builds up across the arcing gap. If the gapis able to withstand this recovery voltage transient, the arc isprevented from reigniting and interruption is completed.

In application Ser. No. 328,656, Lee, filed Dec. 6, 1963, (nowabondoned, but replaced by continuation-in-part application S.N.549,750, 'filed Apr. 20, 1966, and issued as -Patent 3,321,599) andassigned to the assignee of the present invention, it is pointed outthat the current interrupting capacity of the vacuum interrupter can beincreased by applying to the arcing gap during high instantaneouscurrents an axial magnetic field that has its lines of force extendinggenerally parallel to the arc. In order to achieve this improvedperformance, the magnetic eld must be removed or at least reduced to alow strength during the period just prior to current zero and must berelatively strong during the period when the instantaneous current ishigh.

The interrupter of the aforesaid Lee application uses a coil connectedin series with the contacts of the interrupter for developing thedesired axial magnetic field. When the current through the contacts ishigh, this coil develops the desired high strength magnetic field, andwhen the current approaches zero, the magnetic field strength falls tothe desired low value. But unless the eddy currents induced in certainparts of the interrupter by the magnetic field are held to a low value,they can cause the iiux developed by the coil to lag appreciably behindthe current and this will result in a relatively high magnetic fieldremaining across the gap when the current zero point is reached.

One part of the interrupter in which such eddy currents will bedeveloped is the tubular vapor-condensing shield that surrounds thearcing gap. If the shield is a thin metallic member of a highresistivity metal, then the eddy currents induced therein will berelatively low, and only a small amount of flux will be present atcurrent zero. But to improve the vapor-condensing efficiency of theshield, it is sometimes desirable to make the shield relatively thickand of a high conductivity metal, such as copper. In a shield of thischaracter, relatively high eddy currents are induced which can result ina relatively large amount of flux being present at current zero.

An object of our invention is to provide a relatively thick shield ofhigh conductivity metal which is so constructed that the eddy currentsinduced therein by a varying magnetic field extending axially of thearcing gap are limited to very low values. Y Another object is to limitthese eddy currents to low enough values to preclude the maintenance ofa significant axial field at current zero.

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Still another object is to construct the shield in such a manner that itis highly effective in intercepting and condensing electrode vaporsemitted from the arcing gap and is not susceptible to having its eddycurrent-suppressing characteristics impaired by the condensation of suchelectrode vapors on the shield.

For a better understanding of our invention, reference may be had to thefollowing description taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a cross-sectional view through a vacuumtype circuitinterrupter embodying one form of our invention.

FIG. 2 is a cross-sectional view along the line 2--2 of FIG. 1.

FIG. 3 is a graphical representation of the current and magnetic fieldstrength during an interrupting operation.

Referring now to the interrupter of FIG. 1, there is shown a highlyevacuated envelope 10 comprising a casing 11 of suitable insulatingmaterial and a pair of metallic end caps 12 and 13 closing off the endsof the casing. Suitable seals 14 are provided between the end caps andthe casing to render the envelope vacuum tight. The normal pressurewithin the envelope 10 under static conditions is lower than 10-4 mm. ofmercury, so that a reasonable assurance is had that the mean free pathfor electrons will be longer than the potential breakdown paths in theenvelope.

Located within the envelope 10 is a pair of relatively movabledisk-shaped contacts, or electrodes, 17 and 18 shown in their separatedor open-circuit position. When the contacts are separated, there is anarcing gap 22 located therebetween. The upper Contact is a stationarycontact suitably secured to a conductive rod 17a, which at its upper endis united to the upper end cap 12. The lower contact 18 is a movablecontact joined to a conductive operating rod 18a, which is suitablymounted for vertical movement. The operating rod 18a. projects throughan opening in the lower end cap 13, and a flexible metallic bellows 2t)provides a seal about the rod 18a to allow for vertical movement of therod without impairing the vacuum inside the envelope 1li. As shown inFIG. l, the bellows 20 is secured in sealed relationship at itsrespective opposite ends to the operating rod 18a and the end cap 13.

Coupled to the lower end of the operating rod 18a,

suitable actuating means (not shown) is provided for driving the movablecontact 18 upwardly into engagement with the stationary contact 17 so asto close the interrupter. The closed position of the movable contact isindicated by the dotted line 21. The actuating means is also capable ofreturning the contact 18 to its illustrated solidline position so as toopen the interrupter. A circuit-opening operation will soon be explainedin greater detail. A typical gap length when the contacts are fullyseparated is 1/2 inch.

The arc (indicated at 3S) that is established across the gap 22 betweenthe electrodes upon contact-separation vaporizes some of the contactmaterial, and these vapors are dispersed from the arcing gap 22 towardthe envelope. In the illustrated interrupter, the internal insulatingsurfaces of the casing 11 are protected from the condensation ofarc-generated metallic particles thereon by means of a tubular metallicshield generally indicated at 15. This shield 15 is suitably supportedon the casing 11 and preferably isolated from both end caps 12 and 13.Suitable support brackets 19 attached to the outer periphery of theshield 15 are used in the illustrated embodiment for supporting theshield on casing 11. The shield 15 acts to intercept and condensearc-generated metallic vapors before they can reach the casing 11. Toreduce the chances for vapor bypassing the shield 15, a pair of end 3 nshields 16 and 16a are provided at opposite ends of the central shield.These end shields correspond to those disclosed and claimed in PatentNo. 2,892,912, Greenwood et al. assigned to the assignee of the presentinvention.

All of the internal parts of the interrupter are substantially free ofsurface contaminants. In addition, the contacts 17 and 18 areeffectively freed of gases absorbed internally of the contact body so asto preclude evolution of these gases during high current interruption.

Although this invention is not limited to any particular Contactconfiguration, We prefer to use a contact configuration similar to thatdisclosed and claimed in U.S. Patent 2,949,520, Schneider, assigned tothe assignee of the present invention. Accordingly, each Contact is of adisk shape and has one of its major surfaces facing the other contact.The central region of each contact is formed with a recess 29 in thismajor surface, and an annular contact-making area 30 surrounds thisrece-ss. These annular contact-making areas 30 abut against each otherwhen the contacts are in their closed or engaged position and are ofsuch a diameter that the current flowing through the closed contactsfollows a loop-shaped path L that bows outward, as is indicated by thedotted lines of FIG. 1. This loop-shaped path has a magnetic effectwhich tends in a well known manner to lengthen the loop. As a result,when the contacts are separated to form an arc such as 38 between theareas 30, the magnetic effect of current flowing through the loop shapedpath will impel the arc radially outward.

As the arc terminals move toward the outer periphery of the disks 17 and1S, the arc 3S is subject to a circumferentially-acting magnetic forcethat tends to cause the arc to move circumferentially about the centralaxis of the disks. This circumferentially-acting magnetic force ispreferably produced by series of slots 32 provided in the disks andextending from the outer periphery of the disks radially inward bygenerally spiral paths, as is shown in FIG. 2. These slots 32 correspondto similarly designated slots in the aforementioned Schneider patentand, thus, force the current fiowing to or from an arc terminal locatedat substantially any angular point on the peripheral region ofthe diskto follow a path that has a net component extending generallytangentially with respect to the periphery in the vicinity of the arc.This tangential configuration of the current path causes a nettangential force component to be developed which tends to drive the arcin a circumferential direction about the contact.

As pointed out hereinabove, if the interrupter is to successfullyinterrupt the current at a given current zero, it lmust have built upsufficient dielectric strength across the gap between the contacts towithstand the usual recovery voltage transient that appears across thecontacts immediately following the point at which current zero isreached. Whether or not the gap will have this much dielectric strengthis largely dependent upon the extent to which the gap is free of arcingproducts by the time the recovery voltage transient is applied.

The extent to which the gap is free of arcing products depends to animportant degree upon the ability of the interrupter, particularly theshield 15, to condense these arcing products. Ordinarily, no problem isencountered for low current interruptions since the quantity of arcingproducts generated by a low current arc is relatively small. But at highcurrents, much greater quantities of arcing products are generated, andthere is a current level beyond which the interrupter can no longercondense these arcing products fast enough for the gap to withstand therecovery voltage transient.

In the aforementioned Lee application, it is pointed out that thecurrent-interrupting capacity of a vacuum circuit interrupter can bematerially increased by applying to the arcing gap during highinstantaneous currents an axial magnetic field that has its lines offorce extending generally parallel to the arc. In order to achieve thisirnproved performance, the density of the magnetic field must be highduring the period when the instantaneous current is high and must bereduced to a very low level during the period just prior to currentzero. More specifically, when the instantaneous current is high, themagnetic field density in the arcing region must be high enough toproduce a substantial reduction in the arc voltage as compared to thatwhich would be present without the axial magnetic field. J ust prior tocurrent Zero, the density of the axial magnetic field should besufficiently low that there is no substantial impairment of the voltagewithstand ability of the gap at current zero as compared to that of thegap when no magnetic field is present during this interval beforecurrent zero.

The reduced arc voltage that results from the high strength magneticfield appears to result from the tendency that such an axial magneticfield has to confine the arcing products about the arc. By reducing thearc voltage developed during high instantaneous currents, it is possibleto reduce the energy input into the shield during high currentinterruptions. This reduced energy input reduces the temperature rise ofthe shield 15, thus preserving the ability of the shield to rapidlycondense the arcing products generated during high currentinterruptions.

Generally speaking, the higher the arcing current, the greater is thefield strength needed to produce the desired reduction in arc voltage.If the field strength is raised to the desired high level during highinstantaneous currents, then an excessive field strength tends to bepresent just prior to current zero. In application S.N. 328,601,Greenwood and Porter, Vnow Patent No. 3,283,103, filed Dec. 6, 1963, andassigned to the assignee of the present invention, an arrangement isdisclosed for enabling the desired high field strength to be obtainedduring high instantaneous currents without producing an excessive fieldstrength during the period just prior to current zero.

For developing the desired axial magnetic field, which is indicated at50, a coil 52 having its turns surrounding the envelope 10 is providedand is connected in series with the contacts in the power circuitthrough the interrupter so that current fiowing through the arc alsoflows through the coil. During arcing, the circuit through theinterrupter and the coil 52 extends between a pair of opposed terminals54 and 56 via the conductive rod 18a, contact 18, the arc 38, Contact17, rod 17a, connection 57 and coil 52. When current fiows through coil52, it creates a magnetic field S0 which has its lines of forceextending generally parallel to the arc inthe arcing gap.

For controlling the density of the magnetic field in the arcing gap,there is provided an annular iron core 60 that surrounds the casing 11of the interrupter and is disposed between the casing 11 and the coil52. The core 60 is made of a high permeability material such as siliconsteel. Preferably, the core 60 is formed from strips of grain-orientedsilicon steel arranged in stacks 62, circumferentially spaced about theinterrupter casing 11 as best shown in FIG. 2. These stacks 62 are heldin assembled relationship by suitable means including a cylinder 63 ofinsulating material disposed at the inner periphery of the core 60.

When the current through the interrupter and the seriesconnected coil 52is low, the iron core 60 is unsaturated; and because of its highpermeability, the core `60 acts as a flux shunt through which most ofthe liux Vdeveloped by coil 52 is directed so that very little iiuxpenetrates into the arcing gap 22. In other words, most of the flux thatis located radially-inward of coil 52 then follows a path through core60 rather than through the region disposed radially-inward of the core60. When the current through the coil 52 rises to a high value, the coresaturates at a predetermined current level, causing a rapid decrease inits permeability, and thus rendering it ineffective to act as a fluxshunt for fiux produced by current in excess of said predeterminedlevel. A high percentage of this latter ux thus penetrates into thearcing `the core 60. Accordingly,

gap, as is indicated in FIG. 1, and produces an axial eld 50 of highdensity in the arcing gap during lhigh instantaneous currents.

This relationship is illustrated in FIG. 3, where curve F depicts theflux in the center of the contact region during a period of high currentsuch as might result from a short circuit. Such current is depicted incurve I plotted against the same time scale as curve F. The current isdepicted as flowing for a complete half cycle from O to C. Between theinstants O and A, the instantaneous current is relatively low and theiron core 60 is unsaturated. Thus, most of the flux is directed throughthe core,

and very little penetrates into the contact region, as is indicated bythe low fiat portion of the flux curve F between O and A. Following theinstant A, the core 60 begins saturating and the flux created by theadditional current can no longer find a low reluctance path through ahigh percentage of this flux penetrates into the contact region, causingthe flux curve F to rise at a much steeper rate. Shortly after thecurrent reaches its peak, the ux also reaches its peak and then `dropsas the current drops. At the instant B, the current has dropped to alevel that has restored the iron to its vunsaturated condition, thusallowing the iron to shunt most of the flux through a path remote fromthe contact region. Some stray tiux continues to appear in the contactregion after the instant B, but this is a relatively small amount ofliux as is illustrated by the low, relatively fiat portion of the fluxcurve F extending from B to C.

As pointed out hereinabove, it is important that the magnetic fielddensity be reduced to a very low value during the period just prior tocurrent zero. This enables the arcing products to disperse from thearcing gap, thus permitting the gap at current zero to recover itsvoltage withstand ability to substantially the same extent as if noaxial magnetic field had been present during the immediately precedinginterval.

If the flux Awave form had been approximately the -same as that of thecurrent, it will be apparent that the amount of flux at instant B wouldbe a higher percentage of the maximum flux than is the case with theflux wave form F shown in FIG. 3. Thus, the presence of the core 60produces a reduction in the amount of flux appearing just before currentzero for a given maximum value of flux. This permits production of thedesired high values -of flux in the arcing gap during high currentswithout producing excessive flux during the period just prior to currentzero.

It will be apparent from FIG. 3 that the more the flux lags the current(up to about 90 degrees), the higher .will be the flux density duringthe crucial period just before current zero. This lag of the liux behindthe cur- .rent results primarily from eddy currents induced by thevmagnetic field in the conductive parts of the interrupter. .To reducethese eddy currents to a tolerable level, the slots 32 in the contactshave been extended radially in- Ward as far as possible,'and holes 70have been provided in the central region of the contacts, as depicted inFIG. 2. These slots break up the paths for eddy currents induced in thecontact structure by the rapidly changing magnetic field 50, and theholes add resistance to paths that remain. Also the end caps 12 and 13have been .formed of a high resistivity, low permeability material suchas stainless steel in order to limit the eddy currents induced therein.The core 60 is laminated` for the same purpose.

We are concerned in the present application with limiting the eddycurrents induced in the metal shield 15. If this shield 1S consistedonly of a thin-wall tube of high resistivity metal, then the eddycurrents fiowing therethrough during the low ux period around currentzero would be very low. But in the present application, we provide ashield of high-conductivity metal that has a relatively thick wall. Thisconstruction is advantageous in that it more effectively limits thetemperature' rise of the shield during interruption and thus makes itmore efficient as a vapor condenser. But, unless specially constructed,relatively high eddy currents can be induced in a shield of thisconstruction, i.e., with thick walls of high conductivity metal.

Referring more specifically to the shield 15, it will be noted that itcomprises an outer tubular member of a relatively thin-walledconstruction. This outer tubular member surrounds the arcing gap andextends longitudinally of the envelope 11 for substantial distances onopposite sides of the arcing gap. In one specific form of the invention,this outer member is of nickel. Disposed within this outer tubularmember 100 is a tubular internal lining 102 of a relatively thick-walledconstruction. This liner 102 comprises a plurality of arcuate metalsegments 104 spaced-apart circumferentially of the liner to render itdiscontinuous in a circumferential direction. Thelongitudinally-extending spaces between the segments are designated 10S.The segments 104 are preferably made of a highly conductive metal suchas copper.

For fastening each of the segments 104 to the outer tubular member 100,we provide a plurality of arcuate spacers 105 of a material that has ahigh resistivity in comparison to that of the segments 104 and the outertubular member. A preferred material for these spacers is stainlesssteel. Each of the spacers 106 is suitably brazed at its respectiveopposite sides to the outer surface of a segment 104 and the innersurface of the tubular outer member 100. The spacers are located nearthe longitudinally opposed ends of the segments 104.

The axially-directed flux from coil 52 tends to induce eddy currents inthe tubular liner 102 which flow in a direction circumferential of theliner. But the longitudinally-extending spaces 105 between the segmentsprevent these eddy currents from finding a direct path around thecircumference of the tubular liner. While there is still acircumferentially-extending path at each end of the liner 102, this pathis through the high-resistivity stainless steel spacers and thus has avery high resistance. This high resistance severely limits anycircumferentiallydirected eddy currents induced in the liner 102.

The high resistance of this path also results in a much lower timeconstant for any eddy currents in the liner 102, thus causing these eddycurrents to decay more rapidly when the axial magnetic field from thecoil 52 is removed around current zero. This more rapid decay in theeddy currents also contributes to a reduction in the axial lmagneticfield present in the arcing gap around current zero. This time constantvaries inversely with respect to the resistance of the above describedcircumferentially-extending path.

For reducing the magnitude of the eddy currents induced in the outertubular member 100, a plurality of longitudinally-extending slots 108are provided in the outer tubular member. These slots extendlongitudinally of the shield for substantial distances on opposite sidesof the arcing gap 22. The axially-directed flux from coil 52 tends toinduce eddy currents which flow in a direction circumferential oftubular member 100; but, in the central region of the tubular member 100around the arcing gap 22, the slots 108 prevent these eddy currents fromfinding a direct path circumferentially of the tubular member 100. Atthe outer ends of the tubular member 100, there is an uninterruptedcylindrical region providinfy a circumferentially-extending path foreddy currents. But these uninterrupted regions are longitudinallydisplaced so far from the arcing gap that eddy currents flowingtherethrough do not produce a significant axial magnetic field in thearcing gap. The fact that the outer tubular member is of a thin walledconstruction and of a moderately high resistivity material, nickel, aidsin this respect by keeping the eddy currents relatively low.

As statedhereinabove, electrode vapors will be projected radiallyoutward from the arcing gap toward the prevented from traveling throughthe slots 108 by reason vof the fact that the segments 104` arepositioned between the arcing gap and the slots 108 and can thusintercept and condense the vapors before they can reach the slots.

Since the spacers 106 are located behind the segments 104 with respectto the arcing gap, it will be apparent that they are well protected fromthe condensation of arc generated electrode vapors thereon. Bypreventing such vapors from condensing on the spacers and thus forming ahighly conductive coating on the spacers, we maintain their ability tointerpose a high resistance in any circumferentially-extending path foreddy currents induced in liner 102.

Arc-generated metallic vapors will condense on the exposed portions ofthe outer tubular member 100 that are in registry with the spaces 105,but the resultant coating will not short out the spaces. This is becausethe inner surface of the outer tubular member 100 is spaced radiallyoutward from the segments 104. Thus, there is still a space 110 locatedradially outwardly of the segments 104 separating the segments 104 fromthe coating developed in registry with spaces 105.

Although we prefer to construct the liner 102 of a plurality of discretesegments, our invention in its broader aspects is intended to comprehenda construction in which the liner is a tubular member with only a singlelongitudinally extending slot formed therein to render itcirvcumferentially discontinuous.

While we have shown and described particular embodiments of ourinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from ourinvention in its broader aspects; and we, therefore, intend in theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of our invention.

What We claim as new and desire to secure by Letters Patent of theUnited States is:

1. An alternating current circuit interrupter of the vacuum typecomprising:

(a) an evacuated envelope,

(b) a pair of electrodes Within said envelope having a spaced-apartposition defining an arcing gap therebetween across which an alternatingcurrent arc is established,

(c) a tubular vapor-condensing shield within said evacuated envelopesurrounding said arcing gap,

(d) said tubular shield comprising:

(i) an outer tubular metal member surrounding said arcing gap and havingslots therein extending longitudinally thereof for substantial distanceson longitudinally opposite sides of said arcing gap,

(ii) a tubular internal liner for said outer tubular member generallyaligned with said arcing gap and comprising a plurality of discrete.metal segments spaced-apart circumferentially of said liner to rendersaid liner discontinuous in a circumferential direction,

(iii) fastening means for securing said segments to said outer tubularmember in radially spacedapart relationship to said outer tubularmember,

(iv) said fastening means being located between each of said segmentsand said tubular member in a position behind the associated segment withrespect to said arcing gap,

(v) said fastening means being of a material that has a high resistivityin comparison to that of said segments,

(vi) said segments covering said slots to prevent metal vapor from saidarcing gap from passing radially outward through said slots.

Z. An alternating current circuit interrupter of the vacuum typecomprising:

(a) an evacuated envelope,

(b) a pair of electrodes within said envelope having a spaced-apartposition defining an arcing gap therebetween across which an alternatingcurrent arc is established,

(c) a tubular vapor-condensing shield within said evacuated envelopesurrounding said arcing gap,

(d) said tubular shield comprising:

(i) an outer tubular metal member surrounding said arcing gap andextending longitudinally of said envelope for substantial distances onopposite sides of said arcing gap,

(ii) a tubular internal liner for said outer tubular member generallyaligned with said arcing gap and having at least onelongitudinally-extending discontinuity that renders said linerdiscontinuous in a circumferential direction,

(iii) fastening means for securing said liner to said outer tubularmember in radially spacedapart relationship to said outer tubularmember,

(iv) said fastening means being located between said liner and saidtubular member in a position behind said liner with respect to saidarcing gap,

(v) said fastening means being of a material that has a high resistivityin comparison to that of said liner.

3. The interrupter of claim 2 in which:

(a) said tubular member has at least one slot therein extendinglongitudinally of said member for substantial distances onlongitudinally opposite sides of said arcing gap,

(b) and said tubular liner covers any slotsv in said outer tubularmember to prevent metal vapor from said arcing gap from passing radiallyoutward through any of said slots.

4. The interrupter of claim 2 in which:

(a) said tubular outer member is provided with eddycurrent suppressingmeans for imposing a high resistance to currents owing circumferentiallyof said tubular outer member in the region of said arcing gap,

(b) and said tubular liner covers said eddy-current suppressing means toprevent metal vapor from said arcing gap from reaching said eddy-currentsuppressing means.

5. The interrupter of claim 2 in which said fastening means comprisesstainless steel spacers positioned between said liner and said tubularouter member.

6. The vacuum type circuit interrupter of claim 2 in combination with:

(a) means for developing across said arcing gap an ROBERT S. MACON,Primary Examiner.

1. AN ALTERNATING CURRENT CIRCUIT INTERRUPTER OF THE VACUUM TYPECOMPRISING: (A) AN EVACUATED ENVELOPE, (B) A PAIR OF ELECTRODES WITHINSAID ENVELOPE HAVING A SPACED-APART POSITION DEFINING AN ARCING GAPTHEREBETWEEN ACROSS WHICH AN ALTERNATING CURRENT ARC IS ESTABLISHED, (C)A TUBULAR VAPOR-CONDENSING SHIELD WITHIN SAID EVACUATED ENVELOPESURROUNDING SAID ARCING GAP, (D) SAID TUBULAR SHIELD COMPRISING: (I) ANOUTER TUBULAR METAL MEMBER SURROUNDING SAID ARCING GAP AND HAVING SLOTSTHEREIN EXTENDING LONGITUDINALLY THEREOF FOR SUBSTANTIAL DISTANCES ONLONGITUDINALLY OPPOSITE SIDES OF SAID ARCING GAP, (II) A TUBULARINTERNAL LINER FOR SAID OUTER TUBULAR MEMBER GENERALLY ALIGNED WITH SAIDARCING GAP AND COMPRISING A PLURALITY OF DISCRETE METAL SEGMENTSSPACED-APART CIRCUMFERENTIALLY OF SAID LINER TO RENDER SAID LINERDISCONTINUOUS IN A CIRCUMFERENTIAL DIRECTION, (III) FASTENING MEANS FORSECURING SAID SEGMENTS TO SAID OUTER TUBULAR MEANS IN RADIALLYSPACEDAPART RELATIONSHIP TO SAID OUTER TUBULAR MEMBER, (IV) SAIDFASTENING MEANS BEING LOCATED BETWEEN EACH OF SAID SEGMENTS AND SAIDTUBULAR MEMBER IN A POSITION BEHIND THE ASSOCIATED SEGMENT WITH RESPECTTO SAID ARCING GAP, (V) SAID FASTENING MEANS BEING OF A MATERIAL THATHAS A HIGH RESISTIVITY IN COMPARISON TO THAT OF SAID SEGMENTS, (VI) SAIDSEGMENTS COVERING SAID SLOTS TO PREVENT METAL VAPOR FROM SAID ARCING GAPFROM PASSING RADIALLY OUTWARD THROUGH SAID SLOTS.